A transcranial magnetic stimulation is a technique of stimulating neurons by inducing an eddy current or an electric field in the brain through electromagnetic induction. In this technique, as shown in FIGS. 1 and 2, a current (e.g., alternating current) is applied to a stimulation coil placed on the skin of the head to form a variable magnetic field and to induce an eddy current or an electric field in the direction opposite to the coil current in the brain under the effect of the variable magnetic field, and an action potential is generated by stimulating the neurons with the eddy current or the electric field.
FIG. 3 shows an example of a stimulation coil drive circuit. In a principle of generating a momentary current in a coil, first, an electric charge is stored from a power source (including an alternating-current power source, a power circuit, and a booster circuit) into a capacitor. Subsequently, a thyristor is turned on to apply a current to a stimulation coil. The current is applied to a resonance circuit of the stimulation coil and the capacitor through a diode, and the thyristor is then turned off. As a result, a current corresponding to one cycle of a sine wave shown in FIG. 4 is applied to the stimulation coil.
The transcranial magnetic stimulation is used in clinical examinations and neuroscience including measurement of nerve conduction velocity.
In recent years, magnetic stimulation is gathering attention as therapeutic application to neuropathic pain, Parkinson's disease, depression, etc. A drug therapy may not effective to these diseases in some cases and methods of treatment in such a case include applying an electrical stimulation to the brain by implanting an electrode into the brain. However, this method requires a craniotomy and is therefore often not desired by patients. Thus, a repetitive transcranial magnetic stimulation performed by repeatedly applying a non-invasive magnetic stimulation not requiring a surgical operation is being studied as a method of treatment. For example, it is reported that a pain-relieving effect to intractable neuropathic pain is produced for about one day after magnetic stimulation to the primary motor cortex.
However, a conventional magnetic stimulation device weighs about 70 Kg and is available only in well-equipped medical institutions because electrical work is required for installation. Additionally, since a stimulation position is determined while referring to patient's MRI data during actual treatment, the treatment must be performed by a skilled health-care professional. In the treatment of intractable neuropathic pain, a coil is positioned on the target primary motor cortex with accuracy of 1 mm. However, to continuously acquire the pain-relieving effect, a patient must go to a medical institution every day. Therefore, a new magnetic stimulation device for home treatment shown in FIG. 5 has been developed that only requires an operation by a non-healthcare professional for utilization.